Proton therapy, with its characteristic Bragg peak, has a sharp fall-off distally and laterally, potentially lowering dose to organs-at-risk. In prostate cancer, this is most importantly the rectum, as well as the bladder.

Scanning beam proton therapy delivers a series of "spots" of proton radiation. These spots can be utilized in intensity modulated proton therapy (IMPT). Theoretically, IMPT has a better ability than PSPT to conform to a complex target shape.

This study examines if normal tissue sparing can be significantly improved without sacrificing target coverage using IMPT compared to PSPT for prostate cancer patients.

Materials and Methods

15 patients who were treated with PSPT for prostate cancer were evaluated in this study.

The clinical tumor volume (CTV) consisted of the prostate and the proximal seminal vesicles contoured by a single physician.

The dose given to the bladder was improved with IMPT, but the difference was not as pronounced as with the rectum and the anterior rectal wall.

Mean V70 of the bladder for IMPT was 6.7 ± 3.0% compared to 8.2% ± 3.0% for PSPT.

Author's Conclusions

This is the largest presented dosimetric comparison of IMPT versus PSPT for prostate. Large and statistically significant reductions to the rectum and the anterior rectal wall were identified without sacrificing target coverage.

Importantly, this improvement was seen at all dose levels analyzed.

Clinical Implications

The authors use rigorous planning methods including multifield optimization to demonstrate that IMPT has benefit over PSPT in regards to dose to the rectum and the anterior rectal wall.

IMPT has advantages over PSPT above and beyond the dosimetric advantages shown. For example, apertures and compensators are not needed and therefore treatments could be delivered faster and with less physical work by therapists. In addition, IMPT could permit differential dosing and potentially simultaneous-integrated-boost (SIB) to the lobe of the prostate with known gross disease.

It is notable that in this study, only theprostate and proximal seminal vesicles were treated. Chera et al. (IJROPB, 2010)have shown dosimetric feasibility using proton therapy to treat the pelvic lymph nodes for high-risk patients. Additional studies will have to clarify the best treatment technique for these high-risk patients.

The authors do not state how they account for motion which is an important concern in both PSPT and IMPT.

Helical tomotherapy (HT) (Schwarz et. al Radiother Oncol 2011) has been shown to produce similar dosimetric outcomes as IMPT when designed with a SIB with hypofractionation. Further studies will need to be performed to elucidate the relative benefit of HT versus IMPT.

Prospective trials will be required to show a clinical benefit corresponding with the dosimetric benefit seen here.